As oil and gas reserves have been consumed over the years, the extraction of the oil and gas has become increasingly more difficult under more demanding conditions. Accordingly, there is a need for the reserves to be more widely monitored to a higher quality than hitherto, and this is particularly so for oil and gas reserves which lie beneath the sea bed. Optical fibre sensors, together with optical fibre cables to link the sensor to the measurement instrumentation, are being developed for this purpose since they offer specific advantages, particularly in the ability to withstand extremes of high pressure and temperature. Furthermore, such optical fibre sensors may be of a structure and diameter similar to those of the optical fibre cable itself. There are specific advantages in being able to remove and replace a sensor for the purposes of calibration, repair, and enhancement, but this is notoriously difficult to do within the oil industry, particularly with subsea wells because of the harsh environmental conditions and the sometimes considerable distances of tens kilometers between the desired point of measurement at the bottom of a well and the point of use of the information at the production platform. In addition, the substantial demands made on the equipment involved in extracting oil and gas make it very difficult and expensive to solve measurement problems. Hence, it is Very desirable for there to be apparatus which is capable of isolating the problems of measurement from all the other challenges of exploiting the oil or gas reserves, particularly if it is not necessary to install the measurement system at the same time as the other equipment.
The chemical industry has a requirement to monitor discharges from stacks such as chimneys and pipes. In a typical processing site, many stacks will exist over many acres of ground. Currently, this monitoring is achieved by installing ladders and platforms on the outside of stacks, installing expensive equipment on the platforms, and sending trained personnel up the ladders to the platforms to perform measurements on a regular basis. An altogether better approach would be to have an apparatus which is capable of isolating the problems of measurement from the challenge of getting people and equipment into the remote measurement location, particularly if it is not necessary to install the measurement system at the same time as the other equipment.